Over time I have collected some bits and bobs and wish to make at first a simple mono amp. I have done some hunting around the net and reading up and I have very basic understanding of valve amps, and more than enough skill and tools to actually build one following instructions. I have done a search on the forum and have been reading up a bit, theres so much stuff here I would never be able to soak it all up!
I have a few OPT's and power trannies. PS's aren't an issue for me. I would like to try at first a PP 6L6, mainly as I have some nice 6L6GB's and lots of 12AX7's 12AU7's and 12AT7's.
I have found this circuit http://www.audiofanatic.it/Schemi/Tipo/Valvole/finali/pic_finali_PP/6L6PP_12AX7.jpg and can follow most of it but I am not sure of the purpose of the 6.3v link through to the cathode? My OPT's don't have UL taps and most of the simpler schematics on the net seem to use these.
Also I don't seem to be able to hunt down many shematics for ECL86/6GW8 PP setups not using UL OPT's. I think I just got my terminology right
I picked up a faulty kreisler radiogram and would like to make something out of it for small PC speakers or something. The OPT's in it are 10k.
If I have broken any rules or reposted an allready asked question please let me know!
Thanks
I have a few OPT's and power trannies. PS's aren't an issue for me. I would like to try at first a PP 6L6, mainly as I have some nice 6L6GB's and lots of 12AX7's 12AU7's and 12AT7's.
I have found this circuit http://www.audiofanatic.it/Schemi/Tipo/Valvole/finali/pic_finali_PP/6L6PP_12AX7.jpg and can follow most of it but I am not sure of the purpose of the 6.3v link through to the cathode? My OPT's don't have UL taps and most of the simpler schematics on the net seem to use these.
Also I don't seem to be able to hunt down many shematics for ECL86/6GW8 PP setups not using UL OPT's. I think I just got my terminology right
I picked up a faulty kreisler radiogram and would like to make something out of it for small PC speakers or something. The OPT's in it are 10k. If I have broken any rules or reposted an allready asked question please let me know!
Thanks
dsavitsk is correct, it's a cheap and easy way to bias up the heaters. That's really for the benefit of the input stage- the output stage will be far less sensitive to heater-induced noise because of the high level of signal and the suppression of common mode noise in the push-pull stage.
All that said, this circuit sucks. You can find better.
All that said, this circuit sucks. You can find better.
thanks for that, lots of other circuits I have looked at had the cathodes grounded by a resistor.
If this isn't a great circuit, what do you recomend?
I allready have a chassis to experiment with which has 3 octal sockets and 3 nine pin sockets. I have 390v to play with comming out of a 5AS4 in one socket.
The whole idea is to play around with one mono amp, and if I have sucess to build a stereo amp in the future based on what I have learnt, and to possibly drive my quad 57's, but thats probably a few years down the track
I also have one other design aim, and thats to have a few components as possible and to keep it very simple.
A family member interstate suposedly has some 6V6 valves I can get my hands on also so theres another option. But I think my current OPT is probably better suited for PP 6L6, its about 7.5K-8K hand wound, and I have a smaller 8K one from A&R Melbourne.
If this isn't a great circuit, what do you recomend?
I allready have a chassis to experiment with which has 3 octal sockets and 3 nine pin sockets. I have 390v to play with comming out of a 5AS4 in one socket.
The whole idea is to play around with one mono amp, and if I have sucess to build a stereo amp in the future based on what I have learnt, and to possibly drive my quad 57's, but thats probably a few years down the track
I also have one other design aim, and thats to have a few components as possible and to keep it very simple.
A family member interstate suposedly has some 6V6 valves I can get my hands on also so theres another option. But I think my current OPT is probably better suited for PP 6L6, its about 7.5K-8K hand wound, and I have a smaller 8K one from A&R Melbourne.
d1983 said:
I'm not quite as knowledgeable BUT I do know that a 12AX7 type tube alone probably doesn't have the current or voltage swing to drive a pair of 6L6's with any efficacy.
Another reason that this schematic is pretty miserable is that you don't have anything close to a regulated supply for the 6L6G screens, which you HAVE to have. I'm not fully boned up on SS regulators but a decent one shouldn't be too hard to find. The selenium rectifiers just add insult to injury.
Yvesm said:
Thats a pretty cool site, I have printed out a copy of the ECL86 circuit for future reference.
I looked at the 6L6 and wasn't sure about it. i was hoping to avoid all transistors and IC's as in my twisted world they don't belong in a valve amp, Also I don't have a 50 volt winding or similar on either of my power trannies I have which would be big enough to run these valves
Also the voltages for my 6L6GB's look a bit high but I guess that's easily changed for someone with a bit more knowlege . I prefer to use the 12 series pre amp tubes mainly because I have quite a few of them
leadbelly said:
I was told by EC8010 that this won't work well here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=98764
Svein_B said:
CCS LTP can be a superb splitter. For EL84, where not too much swing is needed, a split-load is also an excellent option. In 1955, it was much more difficult to build a CCS, so the diff amp topology was operating under a big disadvantage- that's why a paraphase was an OK choice for the time. Time marches on, progress is made, and thus we have no such limitations.
Here's one that combines both of your tubes - using ECL86 as pre/driver and 6l6 or 5881 tubes as output in a budget single ended design - The Mudlark kit referenced here : http://www.geocities.com/dbr_audio/Parafeed_2x6L6.zip
This has been referenced here before: http://www.diyaudio.com/forums/showthread.php?s=&threadid=64530&highlight=
John
This has been referenced here before: http://www.diyaudio.com/forums/showthread.php?s=&threadid=64530&highlight=
John
An externally hosted image should be here but it was not working when we last tested it.
I have to drop the PS voltage another 50 or 60 volts as its overdriving the type of output valves I have in it, 6L6G's, but I ran i for about 10 minutes and I am amazed it works fairly well. Its got a bad earth mains type buzzing hum through it if its not loaded up but I hope thats just my shoddy construction techniques and the use of both a dodgy old home made output transformer and some resistors and caps pinched from gear over 40 years old. Also many of my capacitor values are slightly differen't, like 22's in place of all the 20's. A few of my resistor values are slightly differen't as well but all within 10 percent. I didn't have a socket for the smaller 6C4 either so its just a heap of connectors pushed over the valve pins and a heap of heat shrink.
It motor boats and then oscilates real high pitched if I have the feedback resistor connected from the output transformer, I cut that line and it works pretty good. But I should have some feedback shouldn't I?
I am absolutely chuffed the damn thing actually works and didn't just explode or catch fire.
I decided to try the schematic below as it looked simple, I understand the basic ideas behind it and I had pretty much all the bits on hand.
An externally hosted image should be here but it was not working when we last tested it.
I had bought an old chassis off ebay and thats what I used for my first try. Its 1am here now so might put more time into it tomrrow
An externally hosted image should be here but it was not working when we last tested it.
d1983 said:
About the only thing that doesn't suck is the Q-Point they specify for the 6L6: Vpp= 360Vdc; Vsgsg= 270Vdc. That gives the best THD performance. As for the rest, the suckitude is pretty manifest.
That's just about the worst possible design for a phase splitter. It'll have considerable imbalance of harmonic distortion between sections, as well as just plain old AC imbalance. Things like that are good enough for "quick 'n' dirty" where audio quality isn't a consideration.
12AX7As just don't have enough drive capability to source enough current to charge the 6L6 grid capacitance at high frequencies, meaning slew rate distortion.
The 0.05uF coupling capacitors aren't big enough. Relying on gNFB to flatten the closed loop low end response does nothing for woofer damping, which relies on NFB for improved damping (lower Zo) but less NFB at low frequencies means less effective feedback and an increased Zo.
The screen PS won't work. You can't use a choke input filter with a half wave rectifier. The regulation is going to be very poor, it's going to be very noisey, degrading the performance of the 6L6s, and presenting a real risk of poofing the 6L6s since the screen voltage is applied before the plate voltage, causing excessive screen currents. Back in "the day" it may have been an acceptable design trade-off when you could get replacement 6L6s at the corner 7 -- 11. Of course, a largely atechnical public didn't realize that replacing the 6L6s every six months wasn't a normal situation.
Looks like excessive gNFB is being used here to cover up for a fundamentally poor open loop design.
Now, Acrosound made some fine OPTs, but it's being wasted on a very inferior circuit.
The whole thing is just:
An externally hosted image should be here but it was not working when we last tested it.
Then what you want is a SET. Google for "2A3" or "300B" amps. This will be considerably simpler than that 6L6 design, and will work much better.
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